1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically, to a method for forming a protecting film on the side walls of a semiconductor device in an atmosphere including energetic particles.
2. Description of the Prior Art
In order to prolong the operating life of a semiconductor device, the semiconductor device is generally covered with a protecting film. In particular, in the case of a semiconductor device having an exposed PN junction at the side walls thereof, for example, a semiconductor laser, a light emitting diode, etc., the side walls should be covered with a protecting film. Unless the side walls are covered, the operating life of a semiconductor laser can be reduced by a so called "facet damage". Accordingly, as the protecting film, an Al.sub.2 O.sub.3 film, an SiO.sub.2 film or a PbO--SiO.sub.2 glass film has heretofore been formed on the facets, i.e. the side walls of a semiconductor laser, by using a vacuum evaporation method, a chemical vapor deposition method or a sputtering method (for example, see 1. Ladany et al, Applied Physics Letter, Vol. 30, No. 2, pp. 87-88 (1977); Y. Shima et al, Applied Physics Letter, Vol 31, No. 9, pp. 625-627 (1977), and; P. A. Barnes et al, Journal of Applied Physics, Vol. 49, No. 5, pp. 2981-2982 (1978).
When the protecting film is formed by the chemical vapor deposition method of by Y. Shima et al, the protecting film covers not only the side walls but, also, the upper surface (i.e. the electrode layer) of the semiconductor laser. In order to cover only the side walls of the semiconductor laser with the protecting film, it is necessary to remove an unnecessary part of the protecting film by an etching method after the formation of the protecting film. Namely, a part of the formed protecting film which covers the side walls of the semiconductor laser is covered with wax, e.g. piccin, and then, the remaining part of the formed film which is not covered with the wax and which covers the electrode layer is removed by an appropriate etchant. However, as a result of this treatment, the surface of the remaining protecting film is contaminated with adhering particles of the wax, and the etchant penetrates between the protecting film and the side walls of the semiconductor laser, so that several properties of the semiconductor laser are frequently deteriorated, for example, the threshold current is increased, and further, the operating life is not prolonged.
Sputtering is well-known to those skilled in the art as a method for forming a protecting film (for example, see L. Holland: Science and Technology of Surface Coating, ed. by Brian N. Chapman and J. C. Anderson, (1974), pp. 369-385, Academic Press London and New York). Sputtering is a preferred method of deposition due to the large area of uniform deposition and the excellent adhesion of a sputtered film. Since the deposition rate of sputtering is low, the thickness control of the sputtered film is precise. In a conventional sputtering method, a protecting film of a semiconductor device is formed by bombarding a target made of a protecting film material with ions of inert gas, to sputter particles of the protecting film material from the target, and to deposit the sputtered particles on the semiconductor device which is located away from the target. Thus, the formed protecting film covers the entire exposed surface including the side walls of the semiconductor device.